1. Field of Invention
This invention relates to electrochemical detection and measurement, and more particularly, to novel means for potentiometrically determining and measuring the presence of zinc ions in solution.
2. Prior Art Statement
Presently the zinc concentration of solutions can be determined spectroscopically and with voltammetric methods. These prior arts do not lend themselves to continuous, real-time monitoring and usually require sampling. Furthermore, these methods will measure either the total amount of zinc or the free and complex bound zinc in a sample. It is especially important in medicine and physiological research to measure ionic activity and this has not been possible with any method known before.
Electrochemical determination of ionic concentrations in solutions has been known for years. In a typical arrangement, a reference electrode and an ion-sensitive electrode are simultaneously immersed into the same body of solution creating an electrochemical cell across which an electric potential develops which is approximately proportional to the logarithm of the activity (or concentration) of the ion to which one of the electrodes is sensitive. A metering device measures the developed potential between the electrodes.
Many different techniques and structures have been developed to provide an ion-sensitive electrode. Of particular relevance to the present application are three patents which describe organic liquid ion exchangers: U.S. Pat. Nos. 3,429,785 issued Feb. 25, 1969 to James W. Ross; 3,445,365 issued May 20, 1969 to James W. Ross; and 3,438,886 issued Apr. 15, 1969 to James W. Ross. In U.S. Pat. No. 3,429,785, an electrode was provided for determining concentrations of ionic species in solution, and particularly for determining the concentration of polyvalent ionic species. The barrier means of this electrode comprises, in continuous phase, an ion-exchanger liquid. Disclosed are several examples of liquid cation exchangers, including normally liquid organophosphoric acids, such as di-2-ethylhexylphosphoric acid and either or both of the mono and di forms of n-butyl phosphoric acid and amyl phosphoric acid. Also disclosed is the use of an appropriate mediator liquid including alcohols with long aliphatic chains in excess of eight carbon atoms, such as octyl and dodecyl alcohols; ketones such as 2-pentanone; aromatic compounds such as nitrobenzene, orthodichlorobenzene, trialkylphosphonates; and mixtures containing phosphonates. These materials were found to produce a barrier means sensitive to polyvalent cations. In particular, an electrode formed of calcium di-2-ethylhexylphosphate as the ion-exchanger in a mediator of dioctyl phenyl phosphonate was found to be sensitive to calcium ions, Ca.sup.+2.
In U.S. Pat. No. 3,445,365, which is a continuation-in-part of U.S. Pat. No. 3,429,785, Ross further discloses the use of means for restricting ion transfer across the barrier in the form of a diffusing membrane between the test solution and the ion-exchanger liquid. The membrane includes channels of finite size filled with liquid exchanger material so that the diffusion coefficient through the channels remains high but the average ion flux through the membrane is considerably less than would occur at a continuous exchanger--test solution interface of the same area as the membrane surface.
In U.S. Pat. No. 3,438,886, Ross discloses a structural variation of the above patents wherein the membrane is situated to serve as a wick which maintains a layer of the exchanger liquid between the reference solution and the solution in which the activity of ions is to be determined. The membrane is preferentially wetted by the exchanger liquid. The resulting electrode is preferentially selective for divalent over monovalent cations.
A large number of membrane materials have been examined during recent years. These have included different glasses for hydrogen-, sodium-, potassium-, and ammonium-ion electrodes, and inorganic salts for fluoride, chloride, bromide, iodide, cyanide, sulphide, copper, lead, cadmium and silver ion electrodes. Liquid state membranes have been employed for potassium, calcium, nitrate and perchlorate ions. The liquid can be soaked into a porous membrane made of filter paper or sintered glass so that a thin layer of organic solvent is formed between the sample solution and an inner solution in the electrode. The organic solvent should be immiscible with water. The liquid layer may also be immobilized by mixing with a polymer like polyvinylchloride or polyurethane. The liquid containing polymer can be cast or pressed into a membrane which is inserted between the sample solution and the inner solution. The liquid used in liquid state membrane electrodes is a mixture between a major component, considered to be the solvent, and a minor component, considered to be the ligand, which forms a complex with the ions to be measured.
Further developments in the liquid ion-exchanger field produced electrodes sensitive to other ionic species, such as: ClO.sub.4.sup.-, Br.sup.-, I.sup.-, NO.sub.3.sup.-, and ClO.sub.3.sup.- in U.S. Pat. No. 3,483,112 issued Dec. 9, 1969; bicarbonate ion HCO.sub.3.sup.- in U.S. Pat. No. 3,723,281 issued Mar. 27, 1973; chloride ion Cl.sup.- in U.S. Pat. No. 3,801,486 issued Apr. 2, 1974; nitrate ions NO.sub.3.sup.- in U.S. Pat. No. 3,671,413 issued June 20, 1972.
In U.S. Pat. No. 3,497,424 issued Feb. 24, 1970 to James W. Ross, an electrode particuarly sensitive to Cu.sup.+2 ions in solution was disclosed and claimed. The ion-sensitive portion of the electrode is a body of an ion-exchanger liquid formed in a water-insoluble salt of S-alkyl thioglycolic acid dissolved in a water-immiscible solvent, such as a 50:50 mixture of decanol and o-dichlorobenzene.
In U.S. Pat. No. 3,691,047 issued Sept. 12, 1972 to James W. Ross and Martin S. Frant, a gel membrane sensitive to calcium ions Ca.sup.+2 was disclosed and claimed. The membrane is formed of a gelled mixture wherein the solid phase is a substantially chemically inert polymeric matrix comprising cellulose triacetate and the liquid phase is an organic ion exchange material such as a salt of a phosphate ester dissolved in a substantially non-volatile solvent such as di-octylphenylphosphonate. Several calcium ion exchangers are disclosed in the patent.
In Analytical Letters, 5(11), 843-850 (1972), D. Ammann, E. Pretsch, and W. Simon published a paper entitled A Calcium Ion-Selective Electrode Based On A Neutral Carrier which describes a synthesized ligand in p-nitroethylbenzene as the membrane ion-sensitive component. The membrane is used in an electrode which measures Ca.sup.+2 ion activity in the range of 10.sup.-1 M to 10.sup.-5 M in unbuffered systems with a selectivity of calcium over sodium and magnesium of 175 and 33,000 respectively.
All of the above references are distinguished from the present invention in that in none of these references is a barrier means (membrane) described in which the composition of the membrane renders the membrane primarily and preferentially selective to the presence of zinc ions, Zn.sup.+2.
Other methods of measuring the concentration of zinc ions in solution have been reported. One indirect selective method using electrodes which measure Zn(II) as tetrathiocyanatozincate has been reported by A. G. Fogg, M. Duzinkewycz and A. S. Pathan in Analytical Letters, 6 (1973) 1101. A similar method measuring Zn(II) as tetrachlorozincate has been reported by R. W. Cattrall and C. P. Pui, Analytical chim. Acta, 87 (1976) 419. These indirect measurements of zinc (II) involve dilution of the original sample which may change the position of the equilibria between zinc-ions and different zinc containing complexes. In this respect the present invention, which measures zinc ion concentration directly, is distinguishable from the indirect methods.
A direct method of measuring zinc ion, Zn.sup.+2, concentration was described by L. Gorton and V. Fiedler in Anal. Chim. Acta, 90 (1977) 233. Using a polymeric membrane electrode and mixing the sample with a calcium--precipitating buffer, the electrode could be made selective for zinc. In this method, dilution of the sample again introduces the possibility that the chemical equilibria between zinc-ions and zinc containing complexes is altered. The present invention overcomes this dilution limitation by providing an electrode applicable for the direct measurement of ionized zinc in a sample solution.